Blood coagulation is the key process involved in both haemostasis (i.e. prevention of blood loss from a damaged vessel) and thrombosis (i.e. the pathological occlusion of a blood vessel by a blood clot). Coagulation is the result of a complex series of enzymatic reactions, where one of the final steps is conversion of the proenzyme prothrombin to the active enzyme thrombin.
Thrombin plays a central role in coagulation. It activates platelets, it converts fibrinogen into fibrin monomers, which polymerise spontaneously into filaments, and it activates factor XIII, which in turn crosslinks the polymer to insoluble fibrin. Thrombin further activates factor V and factor VIII in a positive feedback reaction. Inhibitors of thrombin are therefore expected to be effective anticoagulants by inhibition of platelets, fibrin formation and fibrin stabilization. By inhibiting the positive feedback mechanism they are expected to excert inhibition early in the chain of events leading to coagulation and thrombosis.
Kininogenases are serine proteases that act on kininogens to produce kinins (bradykinin, kallidin, and Met-Lys-bradykinin). Plasma kallikrein, tissue kallikrein, and mast cell tryptase represent important kininogenases.
Kinins (bradykinin, kallidin) are generally involved in inflammation. For example, the active inflammation process is associated with increased permeability of the blood vessels resulting in extravasation of plasma into the tissue. The ensuing plasma exudate contains all the protein systems of circulating blood. The plasma-derived kininogens inevitably will be interacting with different kallikreins, forming kinins continually as long as the active plasma exudation process is ongoing. Plasma exudation occurs independent of the mechanisms that are involved in the inflammation, whether it is allergy, infection or other factors (Persson et al., Editorial, Thorax, 1992, 47: 993-1000). Plasma exudation is thus a feature of many diseases including asthma, rhinitis, common cold, and inflammatory bowel diseases. Particulary in allergy mast cell tryptase will be released (Salomonsson et al., Am. Rev. Respir. Dis., 1992, 146: 1535-1542) to contribute to kinin formation and other pathogenic events in asthma, rhinitis, and intestinal diseases.
The kinins are biologically highly active substances with smooth muscle effects, sectretory effects, neurogenic effects, and actions that may perpetuate inflammatory processes including activation of phospholipase A.sub.2 and increasing vascular permeability. The latter action potentially induces a vicious circle with kinins providing for the generation of more kinins etc.
Tissue kallikrein cleaves primarily low molecular weight kininogen to produce kallidin and plasma kallikrein preferably releases bradykinin from high molecular weight kininogen.